Work machines, such as excavators, backhoes, and other digging machines, are used to excavate or dig holes, ditches, and the like. In some applications, these excavations are required to have a specific depth or be a specific distance away from a given point, such as a wall. In one example, the excavation is for a pipe whose contents must run downhill. The required depth of the excavation could be set forth on a blueprint. In order to determine the depth at which an operator is digging, it is necessary to measure the depth from a known point to the bottom of the excavation. To perform such a measurement, the operator typically touches a bucket tip of the work machine at a first reference point, such as the top of a surveyor's stake, and then moves the bucket tip to a second reference point, which could be located within the excavation. The distance between the first and the second reference points is the offset distance.
In another example, the excavation must be a specific horizontal distance away from a given point, such as an existing structure or a property boundary. In order to determine the horizontal offset distance, and thereby determine where to dig, the operator may touch the bucket tip of the work machine at a first reference point, such as the structure or the boundary line, and then move the bucket tip to a second reference point, where the digging is to occur. Again, the distance between the first and the second reference points is the offset distance.
Some work machines are equipped with a computer system, including software, that is capable of computing the position of the bucket tip. The computer system typically inputs values received from sensors into a kinematics model of the digging linkage to determine its position. For example, U.S. Pat. No. 6,185,493 to Skinner et al. discloses a system for controlling the bucket position of a loader. The system includes position sensors that determine the elevational position of the boom and the pivotal position of the bucket to produce bucket position signals. The system determines the instantaneous position of the bucket.
However, several sources of error may affect the accuracy of the position determined with existing computer systems. For example, if any part of the linkage deviates from the kinematics model dimensions, there will be a discrepancy between the actual position and the measured position of the linkage. One such source of potential error is the flexibility of the linkage components. Gravitational forces or other external forces can cause the digging linkage to deflect from the nominal shape of the kinematics model. Because of the shape of the digging linkage, this deflection may occur in both the horizontal and vertical directions when measured at a bucket tip. Because of the error, the actual linkage position differs from the determined linkage position, in both the horizontal and vertical directions by a deflection value. Because the determined positions are not accurate, a calculated offset distance between the first and the second positions is also not accurate.
This disclosure is directed toward overcoming one or more of the problems or disadvantages associated with the prior art.